1. Field of the Invention
This invention relates to a method for forming a semiconductor device, and more particularly, to a method for forming a protection circuit.
2. Description of Related Art
In order to protect a semiconductor device from the damages caused by electrical surge or over-stress charges, a protection containing a diode, a protection diode, is normally built next to the active region of the semiconductor device.
As shown in FIG. 1A, which illustrates a protection circuit for protecting a P-type metal-semiconductor-oxide (MOS) transistor, a protection diode 12 is connected to the gate of a P-type MOS transistor 10 at its P end, and grounded at its N end. In the case of that a negative input is fed at the input end, the connection to the P-type MOS transistor 10 is closed and the connection to the protection diode 22 is opened. Consequently, the input can go through the P-type MOS transistor 10. Whereas, in the case of that the input at the input end is positive, the connection to the P-type MOS transistor 10 is opened, and the connection to the protection diode 22 is closed. The input is bypassed by the protection diode 12 to prevent the gathering of positive charges at the gate of the P-type MOS transistor 10. Therefore, the gate oxide layer of the P-type MOS transistor 10 is protected from the positive charges gathering at the gate.
Similar to FIG. 1A, FIG. 1B illustrates a protection circuit for protecting a N-type metal-semiconductor-oxide (MOS) transistor. A protection diode 22 is connected to the gate of a N-type MOS transistor 20 at its N end, and grounded at its P end. In the case of that a positive input is fed at the input end, the connection to the N-type MOS transistor 20 is closed and the connection to the protection diode 22 is opened. Consequently, the input can go through the N-type MOS transistor 20. Whereas, in the case of that the input at the input end is negative, the connection to the N-type MOS transistor 20 is opened, and the connection to the protection diode is closed. The input is bypassed by the protection diode 22 to prevent the gathering of negative charges at the gate of the N-type MOS transistor 20. Therefore, the gate oxide layer of the N-type MOS transistor 20 is protected from the negative charges gathering at the gate.
A diode consists of attached N-type semiconductor and P-type semiconductor. Referring to FIG. 2, a semiconductor diode consists of a first-type doped region 30 formed in a second-type well 40. With an additional second-type substrate 50, the diode is able to bypass over-stress charges and voltage to protect a MOS transistor, wherein the first type is N-type and the second type is P-type, or vice versa. Regarding to the valid structure of a protection diode, two protection diodes, which are formed on a substrate with a uniformly electrical property, with different electrical properties can not provide protection to MOS transistors at the same time. Therefore, a conventional method for forming a protection diode can not provide sufficient protection to a dual gate device.
The top view of a conventional protection circuit of a semiconductor device is shown in FIG. 3. Referring to FIG. 3, an active region 110 and the doped region 120 of a protection diode are defined on a substrate 100. A doped polysilicon gate 105 on the active region 110 is connected to the doped region 120 of the protection diode through a metal line 130. The gate 105 is connected to the metal line 130 through a contact 125a, and the doped region 120 of the protection diode is connected to the metal line 130 through four contacts 125b. In the case of that the active region 110 contains a NMOS, the doped region 120 is N-type and formed in a P-well. On the other hand, in the case of that the active region 110 contains a PMOS, the doped region 120 is P-type and formed in a N-well.
The method for forming the foregoing protection circuit includes forming devices and protection diode on a substrate, depositing a dielectric layer to cover the foregoing devices, and forming contacts and metal layer to connect the gate and the protection diode. Since any plasma-related processes preformed before the formation of the metal layer also generate undesired charges gathering on the devices, the undesired charges can not be bypassed by the protection diode through the metal line 130. Therefore, the protection diode fails to prevent the damages on the devices before the formation of the metal line.